Fluids, such as crude oil, molten sulfur and geothermal brine, conducted from subterranean formations through wells contain dissolved or entrained gas, often referred to as "wet gas". Separators are conventionally employed to isolate the wet gas, water and desired crude oil. The isolated wet gas is then converted to a wide variety of transportation fuels and petrochemicals by further separating the C.sub.1 /C.sub.2 dry gas fraction (natural gas) from the C.sub.3 -C.sub.4 and C.sub.5 + paraffin rich fraction. The C.sub.1 /C.sub.2 dry gas fraction is ultimately used as natural gas fuel and is also converted to methanol by steam reforming. The C.sub.3 -C.sub.4 and C.sub.5 + fractions are converted to gasoline, distillate and lubricating oil by thermal cracking.
A commonly observed phenomenon in the production of such fluids as crude oil is the deposition of paraffin or asphaltene onto the walls of production tubing or the wellbore. Such precipitation occurs most readily at decreased operating temperatures and effectively reduces the cross-sectional area of the tubing thereby restricting the flow of the fluid. This problem becomes most severe as the crude flows up the well from the relatively hot reservoir to the relatively cool surface. While wells completed in many reservoirs may flow without artificial lift for several months, or even a year or more, production therefrom will eventually decline to a noneconomic rate due to such reduced reservoir pressures.
Concomitant with the reduced reservoir pressure, the crude becomes more and more viscous as the gas is removed therefrom. The problem is compounded because the lower production rate due to the decreased reservoir pressure increases the residence time of the crude in the production tubing, which in turn results in more heat lost by the crude to the surrounding formations as it travels up the well. This, in turn, increases the viscosity of the oil and further reduces the flow rate. A marked reduction in the flow rate is observed when the temperature of the oil in the wellbore falls below its cloud point and pour point.
Various methods, such as the application of hot oil, hot water and chemicals, have been employed for the removal of paraffin and/or asphaltene deposits from production tubing and wellbores. In particular, hot water has heretofore been employed in a heating string in conjunction with a power fluid system for a downhole hydraulic pump; the hot water and power fluid typically being conducted down the well in separate tubing strings and mixed with the reservoir effluent in the well annulus. However, it has been very expensive to heat the heating string, and to pump a sufficient quantity of the power fluid to the downhole pump at an effective pressure, typically 3000 psi.
While controlling depositions by the addition into the bore of chemicals such as solvents, dispersants, detergents and crystal modifiers has been further documented, such methods are likewise uneconomical since, in general, wells must be pre-treated and production ceased in most cases prior to their introduction.
An object of the present invention is to provide an efficient method and system for recovering subterranean fluids with temperature and/or chemical dependent limited flowability through the use of heated gas as a heat exchange medium in the well which maintains the fluid in a flowable state as it travels to the surface and during processing in surface facilities.
A further object of this invention is to provide a method for descaling a well with chemicals and heated gas.